A conventional DSIA generally comprises a microwave (MW) sensor module, hereinafter a “MW module”, having a microwave transmitter/receiver and a passive infrared (PIR) sensor. For the DSIA to generate an alarm indicating presence of an intruder in a region, hereinafter a “security zone”, protected by the DSIA, generally signals from both the MW module and the PIR sensor must provide evidence of the presence of the intruder. Various algorithms for processing signals from the MW module and the PIR sensor are used for deciding if the signals warrant determining that an intruder has trespassed the security zone. The requirement of “dual detection” for an intruder alarm, as implemented in a DSIA, reduces a probability that the alarm will generate false alarms and improves the reliability of the intruder alarm.
The MW module of a DSIA generates a signal indicating presence of an intruder in a security zone protected by the DSIA responsive to the intruder's motion. The motion causes a Doppler shift in a microwave transmitted by the MW module that is reflected from the intruder back to the MW module. The MW module is usually band limited to a range of Doppler shifts typical of Doppler shifts generated by motion of a human being in the security zone. Therefore, for the MW module to indicate presence of an intruder in the security zone, the Doppler shift must correspond to a “radial” velocity of a body towards or away from the MW module in a range of velocities typical of human motion. In addition, usually the intensity of the reflected microwaves must be greater than a predetermined threshold intensity for the MW module to indicate presence of an intruder.
The PIR sensor of a DSIA usually comprises two (in a “dual” PIR sensor) or four (in a “quad” PIR sensor) IR sensing elements. The IR sensing elements are generally connected in pairs in such a way that the sensing elements of a pair provide opposite polarity signals responsive to IR energy incident on the sensor. The PIR sensor responds to IR energy that reaches the sensor from regions, referred to as “fingers”, of the security zone, which are separated by narrow “dead regions”. The PIR sensor does not respond to IR energy from the dead regions. An intruder moving in the security zone across the fingers causes the PIR to generate a signal having a frequency responsive to a speed with which the intruder moves across the fingers. Signals generated by the PIR sensor are usually band limited to a range of frequencies typical of frequencies generated by motion of an intruder across the fingers of the security zone.
For a signal provided by the PIR sensor to indicate presence of an intruder, the DSIA generally requires that the signal be greater then a predetermined threshold signal. However, the sensitivity of the PIR sensor decreases as the ambient temperature approaches typical surface temperatures of a human intruder. To maintain a substantially constant detection sensitivity of the PIR sensor for the presence of an intruder, the DSIA generally compensates for loss of sensitivity of the PIR sensor as the ambient temperature approaches a “normative” human surface temperature. Compensation is usually achieved by either changing a threshold signal of the PIR sensor or gain of an amplifier that amplifies signals provided by the PIR sensor responsive to the ambient temperature.
A sensor, sensitivity, e.g. PIR sensor sensitivity, refers to sensitivity of the sensor for generating a signal responsive to a stimulus to which the sensor is intended to generate a signal. Detection sensitivity of a sensor in a DSIA, e.g. PIR sensor detection sensitivity, refers to an overall sensitivity of the sensor for generating a signal that the DSIA determines indicates presence of an intruder. The detection sensitivity is a function of sensitivity of the sensor and various criteria, e.g. amplitude thresholds, that the DSIA requires of signals generated by the sensor in order to “accept” the signals as indicating presence of an intruder. Detection sensitivity of a sensor may be adjusted by adjusting the sensitivity of the sensor, adjusting components of the DSIA that operate on signals generated by the sensor and/or adjusting any of the various “acceptability” requirements. Sensitivity of the DSIA for intruder detection refers to an “overall” sensitivity of the DSIA for detecting an intruder.
It is noted that a person's typical surface temperatures can be different for different types of clothing worn by the person and for different regions of the person's body, and is generally different from the usual internal body temperature of a healthy human. Generally, a normative surface temperature for a PIR sensor is determined responsive to the climatic conditions and clothing worn by persons in an environment in which the PIR sensor is intended to operate.
PIR temperature compensation is generally provided for ambient temperatures for which a difference, hereinafter an “operating temperature difference”, between the ambient temperature and the normative surface temperature is greater than a predetermined threshold temperature differential. For an operating temperature difference less than the threshold differential, compensation is not provided and as the ambient temperature approaches the normative temperature, sensitivity of the PIR sensor for the presence of an intruder generally approaches zero. The threshold differential is typically set at between about 1° C. and about 3° C.
The cessation of compensation at the threshold temperature differential reduces the probability that noise, such as for example thermal noise or RF noise, will cause the PIR sensor to generate an erroneous signal indicating the presence of an intruder and thereby the probability that the DSIA generates a false alarm. However, the cessation of compensation also generally results in a reduction in the sensitivity of the PIR sensor and thereby of the DSIA for the detection of an intruder as the ambient temperature of the protected zone approaches the normative surface temperature.
U.S. Pat. No. 5,578,988 describes a DSIA in which a PIR threshold is adjusted responsive to temperature determined by a thermistor to maintain constant PIR sensor sensitivity. The DSIA also comprises a controller that adjusts the PIR threshold responsive to signals generated by the MW module and the MW module threshold responsive to PIR sensor signals. U.S. Pat. No. 5,331,308 describes a DSIA in which if a first one of two sensors in the DSIA continuously generates signals indicating presence of an intruder without confirmation by signals generated by the second sensor, the first sensor is assumed to be generating erroneous signals. The sensitivity of the second sensor is “stabilized” i.e. reduced, so as to reduce a statistical probability of a coincidence between random alarm signals generated by the two sensors generating a false alarm.
U.S. Pat. No. 5,504,473 describes a DSIA in which signals from a MW module and a PIR sensor undergo separate statistical analysis to determine if the signals indicate motion of an intruder, typical noise or faulty operation (i.e. trouble such as circuit failure or masking) of the either of the sensors.
U.S. Pat. No. 6,188,318 describes a DSIA in which signals from a PIR sensor and a MW module are summed. The summed signal is compared to a threshold to determine whether or not an intruder is present in a zone protected by the DSIA. Signals from each of the sensors may be weighted by weighting factors prior to summing. “The weighting factors may be selected to customize the dual sensing intrusion device for optimal detection of an intruder in a given volume of space.” The sum threshold may be determined to discriminate against the DSIA owner's pet.